Plate-shaped (usually planar) waveguides have been used in compact near-eye displays for transmitting angular image information to viewers' eyes as virtual images from image sources located out of the user's line of sight. The image information is generally input near one end of the waveguides and is output near an opposite end of the waveguides. The image information propagates along the waveguides as a plurality of angularly related beams that are internally reflected along the waveguide. Diffractive optics have been used for injecting the image information into the waveguides through a range of angles that are internally reflected by the waveguides as well as for ejecting the image information through a corresponding range of angles for relaying or otherwise forming exit pupils behind the waveguides in positions that can be aligned with the viewers' eyes.
Many of the waveguide displays have been limited to the use of monochromatic light in which the virtual images are formed in a single color. Conventional diffractive optics tend to diffract different wavelengths through different angles, creating chromatic aberrations. Multiple waveguides (e.g., stacked waveguides) or more complex diffractive optics have been used to mitigate these aberrations but current solutions tend to overly limit the number of wavelengths or the different angles through which the image information can be effectively transmitted.
The effective exit pupil size within which the virtual images can be seen in a prescribed position behind the waveguides (i.e., within a designed eyebox) is often overly limited because the diffracted light beams tend to spread apart upon leaving the planar waveguides. This limits the region of overlap within which the virtual image can be seen (i.e., reduces the size of the eyebox). Variations in the diffraction efficiency of the output diffractive optics with position have been used to expand the exit pupils (i.e., eyeboxes) but these variations do not tend to be effective for managing both the multiple angles and multiple wavelengths by which color virtual images are encoded.
Some planar optic displays are also intended to support views of the ambient environment within the same eyebox. This requirement places additional burdens on the output optics to maintain some level of transmissivity.